Since a science has made a considerable progress, various industries have also followed the suit. Particularly, the tele-communication industry has advanced a long stride. In recent years, not only the domestic trunk call can be dialed directly, but also the international trunk call can also be dialed directly. It is rather convenient to the user, but it also raises a lot of annoyances; for instance, (1) when making a domestic trunk call or international trunk call, some money may have been un-necessarily spent because of an inability to shorten the conversation time; (2) some persons often make international trunk calls by using another's telephone set, and that is difficult to prevent. Now, although there are many trunk call control units to be used on the telephone set, their results are still far from being satisfastory because the control units can only control the digit "0" or "1". One who wants to make use of another's telephone can still call thru by dialing an adjacent zone digit first. That conventional trunk call control unit is quite different from the present invention, which uses a micro processor unit to optionally set the number of dialing digits and the talking time.